Factors and applications of IR Spectroscopy

Deals with the factors affecting vibrational frequency in IR spectroscopy and it's applications in pharmaceutical analysis

1. By: Kasturi Banerjee M. Pharm- Pharmaceutics (1 st Year) KLE Society’s College of Pharmacy, Bengaluru Factors Affecting Vibrational Frequency in IR Spectroscopy and it’s applications

2. Factors Affecting Vibrational Frequency in IR Spectroscopy The value of absorption frequency is shifted if the force constant of a bond changes with its electronic structure. Frequency shifts also takes place on working with the same substance in different states (solids, liquids and vapour). A substance usually absorbs at higher frequency in a vapour state as compared to liquid and solid states .  

3. Factors Responsible for Shifting the Vibrational Frequencies from their Normal Values   There are 4 factors:- Coupled vibration and Fermi Resonance Electronic effects Hydrogen bonding Bond angle

4. Coupled Vibrations An isolated C-H bond has only one stretching vibrational frequency whereas methylene group shows two stretching vibrations, symmetrical and asymmetrical. In such cases, asymmetric vibrations always occur at higher wave number compared to symmetric stretching vibration. These are called coupled vibrations since these vibrations occur at different frequencies compared to -CH 2 - group.

5. In case of acid anhydrides, two C=O stretching absorptions between 1850-1800cm -1 and 1790-1745cm -1 with a difference of about 65cm -1 was found. This is due to symmetric and asymmetric stretching. In case of amide (-CONH 2 ), a strong peak is observed which lies between 1563-1515cm -1 . The interaction is very effective probably because of the partial double bond character in the carbonyl oxygen bonds due to resonance which also keeps the system planar for effective coupling. Asymmetric stretching in acyclic anhydride is more intense whereas symmetrical stretching band is more intense in cyclic anhydrides.

6. Requirements: For interaction to occur, the vibrations must be of same symmetry species. There must be a common atom between the groups for strong coupling between stretching vibrations. For coupling of bending vibrations, a common bond is necessary. Interaction is greatest when coupled groups absorb, individually, near the same frequency. Coupling is negligible when groups are separated by one or more carbon atoms and the vibrations are mutually perpendicular .  

7. Fermi Resonance Resonance A vibration of large amplitude produced by a relatively small vibration. Coupling of two fundamental vibration modes produces two new modes of vibration, with frequencies higher and lower than that observed in absence of interaction. Interaction can also take place between fundamental vibrations and overtones or combination tone vibrations and such interactions are known as Fermi Resonance . This phenomenon was first observed by Enrico Fermi in case of CO 2 .

8. If two different vibrational levels, belonging to the same species, have nearly the same energy, a mutual perturbation of energy may occur. Shifting of one towards lower and other towards higher frequency occurs. A substantial increase in the intensity of the respective bands occurs. In this, a molecule transfers its energy from fundamental vibrational level to overtone or combination tone level and back. Resonance pushes the two levels apart and mixes their character, consequently each level has partly fundamental and partly overtone or combination tone character.

9. Example: CO 2 (triatomic) is linear and four fundamental vibrations are expected for it. Out of these symmetric stretching vibration is infrared inactive since it produces no change in the dipole moment of the molecules. In case of aldehyde (-CHO), the stretching absorption usually appears as a doublet due to interaction between C-H stretching (fundamental) and the overtone of C-H bending which is found to be at 2820cm -1 and in case of ketone (-C=O), no such doublet spectrum is found. In cyclopentanone, the absorption due to carbonyl group occurs at 1746cm -1 and 1750cm -1 .

10. Electronic Effects Changes in the absorption frequencies for a particular group take place when the substituents in the neighbourhood of that particular group are changed. The frequency shifts are due to the electronic effects which include:- Inductive effect Mesomeric effect Field effect

11. Inductive Effect The introduction of alkyl group cause +I effect which results in the lengthening or the weakening of the bond and hence the force constant is lowered and the wave number of absorption decreases. Example: Formaldehyde (HCHO) = 1750cm -1 Acetaldehyde (CH 3 CHO) =1745cm -1 Acetone (CH 3 COCH 3 ) = 1715cm -1 The introduction of an electronegative atom or group causes -I effect which results in the bond order to increase. Thus, the force constant increases and hence, the wave number of absorption increases. Example: Acetone (CH 3 COCH 3 ) = 1715cm -1 Chloroacetone (CH 3 CO CH 2 Cl) = 1725cm -1 Dichloro acetone (CH 3 CO CHCl 2 ) = 1740cm -1 Tetrachloro acetone (Cl 2 CH 2 CO CHCl 2 ) = 1750, 1778cm -1

12. Mesomeric Effect They cause lengthening or the weakening of a bond leading in the lowering of the absorption frequency. It is found in conjugated systems. More will be the conjugation, less will be the bond strength and lower will be the wave number. Methyl vinyl ketone Acetophenone Stretch C=O: 1706cm -1 Stretch C=O: 1693cm -1 In some cases, where the lone pair of electrons present on an atom is in conjugation with the double bond of a group, the mobility of a lone pair of electron matters.

13. Example: Benzamide Methyl benzoate Stretch C=O: 1693cm -1 Stretch : 1730cm -1 As nitrogen bond is less electronegative than oxygen atom the electron pair on nitrogen atom in amides is more liable and participates more in conjugation.  

14. Field Effect In ortho substituted compounds, the lone pair of electrons on two atoms influence each other through space interactions and change the vibrational frequencies of both the groups. This effect is called field effect . It is generated due to steric effect . Example: ortho halo acetophenone

15. Hydrogen Bonding It occurs in any system containing a proton donor (X-H) and a proton acceptor. The stronger the hydrogen bond, the longer the O-H bond, the lower the vibration frequency and broader and more intense will be the absorption band. The N-H stretching frequency of amines are also affected by hydrogen bonding as that of the hydroxyl group but frequency shifts for amines are lesser than that for hydroxyl compounds. Because nitrogen is less electronegative than oxygen so the hydrogen bonding in amines is weaker than that in hydroxyl compounds. There are two types of hydrogen bonding:- Intermolecular Hydrogen Bonding Intra-molecular Hydrogen Bonding

16. The H- bonding which is between two different molecules is called intermolecular H-bonding . The H-bonding which is within the same molecules is called intra-molecular H-bonding . Intermolecular H-bonding gives rise to broad bands, while intra-molecular H-bonds give sharp and well defined bands. The inter and intra-molecular bonds can be distinguished by dilution. Intra-molecular H-bonding remains unaffected by dilution and as a result the absorption band also remains unaffected, whereas in intermolecular, bonds are broken on dilution and as a result there is a decrease in the bonded O-H absorption.

17. The strength of H-bonding is also affected by: Ring strain Molecular geometry Relative acidity and basicity of the proton donor and acceptor groups . Examples: In case of amines, the show N-H stretching at 3500cm -1 in dilute solutions while in condensed phase spectra, absorption occurs at 3300cm -1 . In aliphatic alcohols, a sharp band appears at 3650cm -1 in dilute solutions due to free O-H group while a broad band appears at 3350cm -1 due to H-bonded -OH group.

18. Bond Angle It has been found out that the highest stretch C=O frequencies arise in the strained cyclobutanones. The –C-(C=O)-C- bond angle is reduced below the normal angle of 120⁰ and this leads to increased five-character in the C=O bond. Greater S- Character causes shortening of C=O bond and thus C=O structure occurs at higher frequency. Bond angle will decrease, bond strength will increase, vibrational frequency will increase and wave number will increase.

19. Applications of IR Spectroscopy Identification of Organic Compound The identity of an organic compound can be established from its fingerprint region (1400-900cm -1 ). The identity of an organic compound is conformed of its fingerprint region exactly matches with the known spectrum of the compound. The compounds containing same functional group may have similar absorption above 1500cm -1 but they differ in the fingerprint region.

20. 2. Structural Determination This technique helps to establish the structure of an unknown compound. All major functional groups absorbs at their characteristic wave numbers. Example: This IR spectra of amino acids exhibits bands for ionised carboxylic acids and amine salts (- + NH 3 ). No band for free –NH 2 and –COOH groups is observed. + NH 3 -CH-COO - From the IR bands of sulphanilic acid, it is solid that the compound contains + NH 3 and SO 3 - and not free groups as –NH 2 and –SO 3 H.

21. 3. Qualitative Analysis of Functional Groups The presence or absence of absorption bands help in predicting the presence of certain functional groups in the compound. Example : The presence of oxygen reveals that the groups maybe –OH, C=O, -COOR, -COOH, anhydride, etc. But the absorption band is in between 3600-3200cm -1 . The band in this region maybe due to -O-H. In case of –NH 2 , -NH groups , all this can be seen. –NH 2 shows two absorption bands while –NH shows only one band. Its distinction from –OH structure can be made from the extent of H-bonding which is stronger in –OH compounds and causes lowering in wave number.

22. 4. Distinction Between Two Types of Hydrogen Bonding It is known that in H-bonding the electron clouds transfer from a hydrogen atom to the neighbouring electronegative atom. The strength of H-bond is maximum when the proton donor group and the axis of lone pair orbital are collinear and varies inversely to the distance to the distance between hydrogen and oxygen. Example: The hydroxyl compounds in the solid or liquid state exist as polymeric aggregates. The absorption in aggregate form occurs at lower frequencies and bands formed are relatively broad. But when such a substance is dissolved in non-polar solvent such as CCl 4 , the aggregates or polymers break in dimers and monomers. Due to this, the O-H structure absorption shifts to higher frequencies and the peaks below become sharp. This technique helps to distinguish between intra-molecular H-bonding. Ortho nitro phenol exhibits intra-molecular H-bonding. Intra-molecular H-bonded compound doesn’t show any shift in absorption or dilution whereas intermolecular H-bonded does .

23. 5. Quantitative Analysis The estimation of the compound of the mixture can be done by:- Measuring the intensities of absorption bands characteristic of each compound. Knowing the optical density of the absorption band for a pure component. Example: Xylene exists as a mixture of three isomers, i.e. ortho, meta and para xylenes. The percentage composition of mixture can be determined by IR spectrum of the mixture. Bands are formed at: 740cm -1 for ortho isomers 880cm -1 for meta isomers 830cm -1 for para isomers Mixtures of known composition are recorded and the working curves are drawn for the bands.

24. 6. Study of a Chemical Reaction Reduction of a standard aliphatic ketone to form a stronger bond at about 1710cm -1 when it is subjected to reduction, it forms butan-2-ol which absorbs at 3300cm -1 due to –O-H. IR spectroscopy is also used to predict the products formed in a photochemical reaction. Example: When verbenone is irradiated in ethanol solution, the UV absorption maximum due to verbenone disappears and the IR spectrum of crude verbenone appears at 1787, 1740, 1715 and 1685cm -1 . By chromatographic separation we get chrysanthenone, ethyl geraniate,ethyl-3,7- dimethyl octa-3,6-dienoate.

25. 7. Study of Keto- enol Tautomerism Diketones and keto esters exhibit keto-enol tautomerism. They have α-H atom in them. The IR spectrum of such compound contains bands due to C=O, O-H, C=C bonds. Example: Ethyl aceto acetic ester- It exists in keto-enol isomers in equilibrium. The lowering of ν C=O absorption in the enolic bonding form is due intra-molecular H-bonding which is stabilised by resonance. The appearance of bands clearly confirms keto-enol tautomerism in aceto acetic ester.

26. 8. Study of Complex Molecules This technique is also useful to establish the structure of complex molecules. Example: Two structures of penicillin were prepared on the basis of IR spectral. β-lactam Oxazolone

27. The IR structure of oxazolone shows two characteristic bands: 1825cm -1 due to ν C=O ν C=N due to 1675cm -1 It is found that no such band appear in the spectrum of penicillin. Thus, oxazolone structure for penicillin is ruled out. It is known that β- lactams do not absorb near 1770cm -1 whereas β- lactam fused to thiazolidine ring exhibits a band at 1770cm -1 . Thus, the β- lactam structure of penicillin is confirmed.

28. 9. Conformational Analysis Useful for conformations of cyclic compounds cyclohexane exists in boat form and chair form. Chair Form Boat Form There are 18 IR active C-C structure and CH 2 rocking and twisting vibration for boat form (II) whereas there are only five for the chair form (I).

29. The spectral examination of cyclohexane in the region 1350-700cm -1 reveals five bands expected for chair form. This shows the greater stability for chair conformation over boat conformation. By IR spectroscopy, axial and equatorial substituents in cyclohexane substituents in cyclohexane can be distinguished. The equatorial substituent usually absorbs at a higher frequency than does the same substituent at axial position. This is due to steric hindrance of C-X bond with adjacent H-atoms.

30. 10. Detection of Impurity in a Compound IR spectroscopy is also useful in the detection of impurity in a compound by comparing its spectrum with the spectrum of the authentic sample of the compound. Pure sample always consists of poor bands and also some additional bands.

31. Reference: Instrumental Methods of Chemical Analysis, B. K. Sharma, page no: 271-276. Instrumental Methods of Chemical Analysis, Gurdeep R. Chatwal, Sham K. Anand, page no: 2.55-2.59 . Instrumental Analysis, Douglas A. Skoog, F. James Holler, Stanley R. Crouch, page no: 505-529.

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